[EMAIL PROTECTED] wrote: > In the case of the DTV-Utah facility on Farnsworth peak, each station > has its own transmitter, but they're all fed into a big combiner which > then feeds a common antenna. (which may be a distinction appreciated > only by TV-transmission nerds. :-) > > Anyway, at both my home and office I've observed a similar problem in > that some OTA ATSC stations come in like gangbusters while others are > poorly received, if at all. Knowing that the signals are literally > coming out of the same antenna, I was initially baffled by this. The > best explanation I've heard is that the problem isn't a matter of a > weak signal, it's a matter of a poor signal-to-noise (S/N) ratio, > which is greatly affected by destructive interference due to multipath > which, in turn, varies highly with both frequency as well as the > geographic locations of the transmitter and receiver. In other words, > in any given spot, a receiver might have fabulous S/N ratios on > several stations but lousy S/N ratios on other stations. Here along > the Wasatch front, where most DTV signals come out of the same > antenna, the only difference for any given receiver location is the > frequency. Different frequencies at any given receiver location will > have different patterns of destructive interference. On analog TV, > multipath is often "seen" as ghosting in the picture. On digital TV, > multipath doesn't cause ghosting in the picture; it reduces the > quality of the digital data stream being received--sometimes to the > point that the data can't be recovered at all. Also keep in mind that > multipath and destructive interference can be worse on UHF than VHF > because of the higher frequencies; higher frequencies mean that > smaller differences in path length can add up to bigger destructive > interference problems. The mountains all around us don't help; I was > surprised when I heard that "dirt" can reflect a UHF TV signal, but > I'm told it does--very effectively. :-) Anyway, if your ATSC receiver has > a S/N meter (mine does), check it out and see if it correlates with > the poorly-received OTA ATSC signals. > > If you're in a position where you can "see" Farnsworth peak, then it's > highly unlikely that a "weak signal" is the cause of any reception > problem; it's much more likely that your antenna is receiving several > "copies" (reflections) of the problem signal--each "copy" of which is > separated in time based on the differing path lengths. Your antenna > will be receiving the strong "direct" signal as well as some number of > slightly weaker reflections, and those reflections tend to > destructively interfere with the direct-path signal. About the only > thing you can do in such circumstances is use an antenna with a more > "directional" pattern; it's not overall "gain" that is of interest, > but the degree to which side and back lobes have significantly lower > gain than the (hopefully narrow) forward lobe which is pointed towards > the transmitting antenna. > > Chris
Chris, You have some great information here, though I "recommend" fewer quote marks :) . Just for the fun, might I recommend reading the gallery of "misused" quote marks? http://www.juvalamu.com/qmarks/ Brandon Stout http://mscis.org /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
